Unit 5 Notes #4 - Phylum Platyhelminthes: Flatworms
A) Body Plan: - All have bilateral symmetry.
-Exhibit Cephalization: Concentration of sensory
organs and nervous tissue at the anterior end of the
-Well-developed third tissue layer called the mesoderm
between the ectoderm and endoderm.
-Mesoderm includes: reproductive, excretory, and
-Possess an incomplete gut: single opening (mouth/anus)
to the gastrovascular cavity; use mouth to ingest food
and use mouth to expel undigested wastes.
1. Class Turbellaria: -Free living
-Incomplete gut -No suckers or hooks
-Ciliated epidermis Example: Planaria
2.Class Trematoda: -Parasitic
-Incomplete gut -Suckers
-Outer Cuticle (tegument) Example: Flukes
3. Class Cestoda: -Parasitic
-No gut -Suckers and hooks together form a
scolex for attachment to host.
-Body consists of repeating sections called proglottids.
C)Adaptations Common To Most Parasitic Flatworms:
1) Parasitic flatworms often have suckers and hooks for
attachments, form a structure called a scolex.
2) Outer cuticle (tegument) for protection so as not to be
digested or destroyed by the host.
3) Loss of digestive system in some (tapeworms) – some
will saprophytically absorb nutrients through ectoderm.
4) Complicated life cycle with the production of many
eggs and/or offspring and use of many hosts to ensure
D)Characteristics Of Common Platyhelminthes Members:
a. Digestion: Feed on small animals and the remains of
larger dead animals.
-Mouth/pharynx is located mid-way along the ventral
surface (not near the head) it acts to push food into the
-Digestive glands are present in the pharynx and
gastrovascular cavity (they secrete enzymes to digest
-Gastrovascular cavity will also circulate nutrients and
oxygen to various parts of the body.
-Indigestible wastes are eliminated out of the mouth.
b. Nervous : -Most primitive animal with a real
-A pair of ventral (along belly) nerve cords run the
length of a nervous system.
-Connect to a pair of small brains or ganglia (a group of
nerve cells) at the anterior end.
-Show cephalization with a variety of sensory cells.
-Sensory cells are sensitive to touch, light, and various
c. Muscles: -Three layers of true muscle are located
below the ectoderm.
-The outer layer of circular muscle that constrict the
worm, making it longer and thinner.
-Next layer uses longitudinal muscles to shorten the
-There is also a third layer that runs dorso-ventral
(back to belly) and makes the worm a little flatter.
d. Locomotion: -Body moves by co-ordinating its
muscles and gliding over a mucous trail that it secretes
from glands in the epidermis.
-Cilia on the ventral epidermis assists in gliding.
a. Digestion: -They are parasitic, usually in the
blood, liver, or intestine of a variety of animals.
-Mouth present in the middle of the anterior (oral)
-The pharynx swallows the host’s tissues and bodily
fluids (blood) into its gastrovascular cavity.
b. Circulation and Respiration
-Gastrovascular cavity for circulation and respiration
-They live in tissues that are well supplied with
nutrients and oxygen by the host’s blood.
c. Nervous: - Similar to Planaria but they lack most of
the special sensory cells found in Planaria.
d. Muscle: -Similar to planaria, but lack cilia on the
a. Digestion: -Have no mouth or gastrovascular cavity.
-Attach themselves to the lining of the host’s intestine
by suckers and hooks (Scolex)
-Worm feeds by absorbing digested food.
- Has a modified epidermis called a Tegument, this
thicker skin prevents them from being digested.
b) Circulation: -Receive oxygen from blood vessels in
the host’s intestine.
-When oxygen is not available, they can live without it.
E) Reproduction In These Three Common Members of
1. Planaria: -Reproduce asexually by fission (splitting)
-Missing parts will regenerate.
-Can also reproduce sexually.
-Hermaphrodites (contains both sexes in one
individual), but requires reciprocal exchange of sperm
and eggs between two individuals.
2. Flukes: -Also hermaphrodites.
-Complex life cycle with numerous larval stages that
infect a number of hosts.
3. Tapeworms: - Constantly budding new body sections
(proglottids) posterior to its scolex.
-Each proglottid contains both male and female
-Mature proglottids with fertilized eggs detach and pass
out of the host with the feces.
-Often have larval stages that infect a number of
Advances of the Platyhelminthes Over the
1. Platyhelminthes have their tissues organized into
organs and even have some simple systems.
2) Unlike the previous two Phyla (Porifera and
Cnidaria), the Platyhelmithes have 3 embryonic layers
with the development of the middle layer (the
mesoderm). The mesoderm has also provided for better
muscle development and thus has resulted in an animal
that moves around more efficiently.
3) As a result of moving around, platyhelminthes have
further developed cephalization.
4) Cnidarians had a nerve net for conducting impulses,
but the platyhelminthes have centralized the nervous
system into two longitudinal nerve cords. (faster
transmission of nerve impulses).